Police Chief Jeri Williams said at a news conference late Tuesday night that the police response was prompted by a protester who had thrown a gas canister at police providing security outside the rally.

Several witnesses said they saw a few water bottles tossed at officers.

Two of the arrests were for aggravated assault on officers, one was for criminal damage and one was for an outstanding warrant.

Protesters were outside the convention center several hours before the rally started at 7 p.m. It wasn't until after the president's event ended, about 8:30 p.m., that the confrontation with police apparently escalated.

Williams said police would continue to investigate the incident and provide a complete report later.

Williams said no officers were injured in the protests, but two were hospitalized for heat exhaustion.

Fire officials gave a final count of 56 people treated and 12 transported to the hospital for heat-related illness. No civilians were treated for injuries related to the protest.

Both Williams and Mayor Greg Stanton said the rally was "successful."

Thousands of people were in downtown Phoenix for the rally and protests, they said, and only a small number were involved in the confrontation with police.